Aircraft frequently use fuel sensors located within the fuel tank in order to monitor fuel levels. The installation of equipment into a fuel tank is normally achieved in one of two ways: full internal installation or through wall installation. Those components that are fully installed inside the fuel tanks require access to the interior of the fuel tank during installation and during maintenance operations. Through wall fuel sensors have the benefit of allowing servicing of the fuel sensor from outside of the fuel tank.
The installation of equipment through the wall of a fuel tank can be done for both permanent and removable components. Permanent components are not intended to be replaced or internally inspected during maintenance (e.g., water drains). Removable components allow servicing and replacement of parts from outside the tank without the need to drain it. In order for removable components to be replaced, the removable components must have a way to prevent the fuel from leaking out of the tank while the component is being serviced and/or is completely removed (e.g., to be replaced).